Should jet engines be fixed or not,ever?

[camlost]

Now that Squad is finally reviewing their aerodynamics, I feel it's about time to stress the problem with jet engines. Flying airplanes in stock KSP is truly weird, but 'souposphere' is only half the concern. The erroneous jet engine/inlet model ruins the game no less, yet not talked about as much as the aerodynamics.

Initially, all engines in KSP were rockets, they have a max thrust and a curve of altitude vs Isp, a simple model that works fine. And later on, jet engines are added as derivation of rockets, on top of the Isp curve, a speed vs thrust curve is added. Jet engines also requires another resource named IntakeAir, that is created by inlets.

This model is totally wrong and leads to terrible mistakes in two aspects. First, thrust is dependent not only on speed, but also on altitude (roughly proportional to static air pressure). Also, Isp should be dependent on speed besides altitude. Second, when we talk about Isp (usually 2000-4000s range for jet engines) we are referring to the consumption of fuel, not including the air going through the engine. However, in KSP the IntakeAir is also counted as a fuel. Take the 'basic jet' part for example, the actual Isp is 15x higher than what the game says.

Players are heavily misled by these problems. Because thrust doesn't decrease with altitude, basically any design can reach high speed and altitude with enough wings and controls. As long as there's enough IntakeAir, the engines can push the aircraft all the same, and when IntakeAir runs out, they suddenly flame out. The unusually high Isp makes a player never care about the fuel. The result is clear: it encourages engine spamming and inlet spamming, that's why you see them so much. It also makes hypersonic flight a trivial task, which it really shouldn't be.

Why nobody cares then? Maybe I need a PR campaign? I think the reason lies in how things are balanced: people would just make a plane and say 'hey it flies' or 'hey it SSTOs' and be done with it. Hopefully applying mistakes on top of other mistakes (aka souposhpere) makes things right? I don't think so. Bugs are bugs not features and they should be fixed.

Now that they are finally redoing the aerodynamics, maybe it's time to review the jet engines as well. It's a shame that these problems goes unnoticed forever.

[Wanderfound]

Would I like to see the jets improved, bringing them closer to reality? Yes. Will I be hugely bothered if they aren't? Not so much.

The aero stuff is in your face and impossible to ignore while flying; the engine stuff, for me at least, doesn't break immersion to anywhere near the same level.

But, yeah: if I was lord and master of KSP, I'd get rid of the magic ponydust turbojets and replace them with a broader range of more realistic jets. Perhaps not to full AJE degrees of detail, but with reasonable fuel consumption levels and a separation between turbofan/turbojet/ramjet/scramjet, and a fix for the problem of "your jets are a heavy point-source stuck on the back of the plane".

There was a proposal a while ago to redistribute some mass from engines to intakes, to give the jets a more realistic weight distribution: http://forum.kerbalspaceprogram.com/threads/92819-An-arguement-for-a-simple-intake-jet-engine-tweak

Sounded like quite a good idea to me.

Edited January 14, 2015 by Wanderfound

[GoSlash27]

The stock "turbojet" shouldn't exist. The basic jet does a good job of simulating a high performance turbojet, while the turbojet is ridiculously good.

I would reconfigure the turbojet as a "ramjet". Works the same at supersonic speeds, but won't work at all at subsonic speeds.

Best,

-Slashy

[chaos_forge]

I have a question: If the ISP code is fixed so that lowering ISP lowers thrust instead of increasing fuel consumption, would that fix the "jets function the same at all altitudes" problem? Because, if so, then fixing the ISP code could fix the incorrect behavior of both rockets and jets in one fell swoop.

The unusually high ISP seems like it could be fixed by simply dividing the current ISP by 15, and wouldn't require changes to anything other than part configs.

Edited January 14, 2015 by chaos_forge

[Kegereneku]

Not really, engine design are quite different from rocket engine. For similar reason but following a different mechanic

See this thread : http://forum.kerbalspaceprogram.com/threads/105041-Jet-engine-Intake-velocity-ranges

(sponsored by me)

Edit : but it doesn't mean it wouldn't be cool for SQUAD to "fix" the calculation. At least if it doesn't make design unintuitive. It IS a realistic effect but people don't intuitively know why and could think it is a bug.

Edited January 14, 2015 by Kegereneku

[camlost]

Not really, engine design are quite different from rocket engine. For similar reason but following a different mechanic

See this thread : http://forum.kerbalspaceprogram.com/threads/105041-Jet-engine-Intake-velocity-ranges

(sponsored by me)

Edit : but it doesn't mean it wouldn't be cool for SQUAD to "fix" the calculation. At least if it doesn't make design unintuitive. It IS a realistic effect but people don't intuitively know why and could think it is a bug.

No, that would be even worse. What really should be is that the Isp stays roughly the same, but thrust decreases in high altitude, so less fuel is burnt. That's why passenger planes always cruise at 30000 feet.

Divide the Isp by 15 has a huge impact on plane designs, and everything especially fuselage has to be re-balanced.

the engine stuff, for me at least, doesn't break immersion to anywhere near the same level.

Maybe because jet engines are difficult and rarely taught. We've seen home-made rockets but there's no home-made turbojets. However, that is no excuse to ignore the issues. The two points I've made are as bad as the aerodynamics, has been there for as long as the aerodynamics, yet nobody realizes how serious it is. All these proposals about re-balancing stuff completely missed the point, instead they need a complete overhaul, as badly as the aerodynamics.

[Kegereneku]

That's not really why jet-liner fly at high altitude, but I'm too tired to go in explanation when you can google it.

Let's just say, equivalent to Rocket, jet-engine are made specifically to be best efficient at some altitude (but also at some speed). If you are out of this range it doesn't work as efficiently.

With the exception of Aerospike which were meant Specifically to keep the same thrust at any altitude. Unfortunately since KSP don't use the same equation its core-quality isn't being used right.

The Devs certainly know what is unrealiste in their game, I wouldn't doubt of it from the telling of HarvesteR, but they must do compromise.

[DundraL]

I too would like to see a few changes to jets... mainly the one Wanderfound mentioned (I'm surprised you remember that... still pushing for it though! Bump it if you want!)

But I definitely see that their ISP is currently OP and could use a good rework.

Perhaps making jet engine heat production scale according to an intakes speed rating? That would at least provide a reason to differentiate between high speed and low speed intakes, as well as passive VS active air compression.

I DEFINITELY like the idea of scaling thrust with amount of available intake air... OK I think I just need to stop or make an intake rework suggestion thread....

Edited January 15, 2015 by DundraL

[camlost]

The Devs certainly know what is unrealiste in their game, I wouldn't doubt of it from the telling of HarvesteR, but they must do compromise.

Really? I highly doubt that, and I never seen any dev mentioning what I said. Ignoring such serious issues for more than two years and not touching it is not a compromise.

[rkman]

I never seen any dev mentioning what I said.

I share your concern about jet engines, but i don't think the fact that the devs have not mentioned it means much. They are under no obligation to address our every concern, and they have released things that they had not mentioned and that we hadn't even thought of.

[Slugy]

Entirely agree that jet engines need a good look at, for the reasons stated, but this seems incorrect.

Because thrust doesn't decrease with altitude, basically any design can reach high speed and altitude with enough wings and controls. As long as there's enough IntakeAir, the engines can push the aircraft all the same, and when IntakeAir runs out, they suddenly flame out.

The thrust does decrease with altitude: a "basic" jet engine will never take you to orbit (or anywhere near) as it lacks the needed thrust at altitude to keep flying.

I do suspect that while looking at aerodynamics jets and intakes will have to be looked at.

[camlost]

Entirely agree that jet engines need a good look at, for the reasons stated, but this seems incorrect.

The thrust does decrease with altitude: a "basic" jet engine will never take you to orbit (or anywhere near) as it lacks the needed thrust at altitude to keep flying.

I do suspect that while looking at aerodynamics jets and intakes will have to be looked at.

The thrust doesn't decrease with altitude, you cannot find such curve in cfg. In 0.22 version or something, Squad added a function to auto-throttle-down the engines when IntakeAir is not enough.

That is wrong and totally missed the point. Engines need inlet to work, but simply adding more inlets will not make engine work better at high altitude.

They claimed to have done it to improve game experience. However this is the exact problem that encourages intake spamming.

My point again: IntakeAir must die. Using IntakeAir as a resource is unreal, counter-fun, and encourages stupid things.

[prophet_01]

I absolutely agree that there is something srsly wrong with the current gameplay. Jet engine booster stages for rockets feel rly wrong...

I'm playing with FAR, DRE and b9. The result is that the isp is nerfed a little and jet engines overheat at high speed (best you can get is about mach 3). Although that doesn't fix the problem entirely or is all that much more realistic (refering to the big picture) it is a good aproximation imo. It bans the most obvious exploits that I see with the stock gameplay. Sure, it is still possible to spam intakes, but it won't get you much closer to orbit due to overheating. So a mini SSTO or a 60% payload fraction are mostly off the table. I'm more or less willing to live with the messed up isp, although variable eficiencies would certainly be great.

My knowledge is mostly based on wikipedia and forum posts from people that obviously know way more about this topic, so It's not likely I missed something. If so, tell me what nonsense I should stop to spread around

@slashy I would prefer the ram engine aswell. However, the SR 71 is a working plane that gets into the air with another type of engines due to it's design specification (sry english is not my native language and I struggle to find the right technical phrases here). Anyway this particular plane shows pretty well that it is possible to use (modified) ram engines only. I'm fine with the aproximation that turbo jets in ksp work abou the same. If I missed something here, please tell me

Edited January 16, 2015 by prophet_01

[Kegereneku]

As said earlier, rocket engine don't necessarily become better with altitudes. Hourglass shaped nozzle are typically shaped to work best (more thrust) at a given pressure (from Ground to Zero). So you have rocket engine that are best for lift-off and other best for space-travel.

SQUAD reflected this by cheating through the fuel consumption, it is unrealistic but it mostly do its job at distinguishing engine per use. The problem is for Aerospike which where meant to be awesome at any pressure but can't because all engine have its quality.

Now for the case of jet-engine they are also meant to work best at specific altitude, but in a different way.

More explanation on a suggestion topic of mine.

The source of "intake spamming" problem isn't caused (to my opinion) by the fuel-hack but the attempt to balance jet engine and keep them from being overpowered but working up to not exactly 70km.

Since the limit isn't clearly established and because we can, people will spam air-intake

It do however reflect realistically that reaching Orbital speed with air-breathing engine is quite frigging hard.

[prophet_01]

In stock ksp you can spam intakes and "airhog". So basically fly at 2000 m/s and do a suborbital hop. A rly small burn with a tiny rocket engine at apoapsis is usually enough to get an orbit. I've seen lets plays with jet only hops to a 200km apo/ 40km peri almost-orbital hop. Rly unrealistic...

[Scoundrel]

The engines are fine, IMHO. All that needs to be added is that inlets acquire heat as velocity increases until they explode or are closed, and the velocities at which heat is accumulated varies with each inlet. You can even do the same thing for jet engines if that wasn't enough. A mod would suffice, but I wouldn't mind seeing it in stock.

That said, I find it interesting that people have issues with how other people play a game. Things like the "problem" of intake and engine spamming, because [insert personal opinion and rationalization of why that personal opinion validates dictating what game experience other players should have].

It makes me think of people complaining about how other people play solitaire.

[Jouni]

The real problem is that stock turbojets are currently the best rocket engines for launching payloads less than 100 tonnes to orbit. Actual rocket engines can't compete, because turbojets have a high enough TWR and Isp around 40000 s. They should either be renamed to Magic Star Wars Engines, or their TWR should be nerfed to below any rocket engine and their fuel usage increased by 16x.

[GoSlash27]

@slashy I would prefer the ram engine aswell. However, the SR 71 is a working plane that gets into the air with another type of engines due to it's design specification (sry english is not my native language and I struggle to find the right technical phrases here). Anyway this particular plane shows pretty well that it is possible to use (modified) ram engines only. I'm fine with the aproximation that turbo jets in ksp work abou the same. If I missed something here, please tell me

Prophet,

One important thing:

The SR-71 tops out at about the same speed that the basic jet does; Mach 3. This is why I say the "basic jet" is a good approximation of a high performance afterburning turbojet and the "turbojet" has unrealistic performance.

Best,

-Slashy

[camlost]

In stock ksp you can spam intakes and "airhog". So basically fly at 2000 m/s and do a suborbital hop. A rly small burn with a tiny rocket engine at apoapsis is usually enough to get an orbit. I've seen lets plays with jet only hops to a 200km apo/ 40km peri almost-orbital hop. Rly unrealistic...

Yes. That's why IntakeAir needs to be got rid of.

The engines are fine, IMHO.

That said, I find it interesting that people have issues with how other people play a game.

IMO? I only stated facts. I have zero interest in your opinion or how you or anybody play. I, as a customer, have problem with how KSP is designed.

The real problem is that stock turbojets are currently the best rocket engines for launching payloads less than 100 tonnes to orbit. Actual rocket engines can't compete, because turbojets have a high enough TWR and Isp around 40000 s. They should either be renamed to Magic Star Wars Engines, or their TWR should be nerfed to below any rocket engine and their fuel usage increased by 16x.

Yes, you can even use jet engines as first stage. Won't happen with a correct thrust/altitude relationship. In reality, people refer to TWR at sea level when they talk about airplanes. It's probably fine to have a TWR=15 turbojet, but thrust really needs to change with altitude properly.

Prophet,

One important thing:

The SR-71 tops out at about the same speed that the basic jet does; Mach 3. This is why I say the "basic jet" is a good approximation of a high performance afterburning turbojet and the "turbojet" has unrealistic performance.

Best,

-Slashy

It is not a good approximation of anything, because of the two problems I stated.

[Kegereneku]

Scoundrel, I fail to see how your idea would solve the problem (pre-aero-rework) or improve the gameplay.

It put an upper limit to their speed (through heat) but in a way that favor minmaxing (lifting to orbit as fast as possible) rather than their expected use (slowly increasing the speed)

Also, inlet/intake that explode is also hardly intuitive (or realistic actually), engine however...

Goslash27, this is really just a naming problem. Basic jet engine are also turbojet but the turbojet have have the quality of a turbojet and a scramjet.

I hope I'm right in thinking the devs will make a third engine and rebalance (with or after the aero rework) engine so reaching orbit do require engine that don't work at low speed.

[Red Iron Crown]

In my opinion the fuel consumption of the jets should be corrected, they're way too fuel efficient. I'm less convinced about the velocity curve, while it is unrealistic it is also great fun, for me at least.

As said earlier, rocket engine don't necessarily become better with altitudes. Hourglass shaped nozzle are typically shaped to work best (more thrust) at a given pressure (from Ground to Zero). So you have rocket engine that are best for lift-off and other best for space-travel.

Interesting, I've never heard of a RL rocket engine that has a lower Isp at low atmospheric pressure than in high, the most I've seen is ones that are not as bad in atmo in exchange for being less good in vacuum but vac Isp is till greater than atmo Isp. Do you know of an example of such a design?

[camlost]

In my opinion the fuel consumption of the jets should be corrected, they're way too fuel efficient. I'm less convinced about the velocity curve, while it is unrealistic it is also great fun, for me at least.

Question is, did you have great fun because of it or in spite of it?

[Red Iron Crown]

Because of it, I think. Being able to get up near orbital velocity on jet engines is the fun part for me. My rational brain is screaming about how silly and unrealistic it is while my pleasure centers are fully activated.

[camlost]

Because of it, I think. Being able to get up near orbital velocity on jet engines is the fun part for me. My rational brain is screaming about how silly and unrealistic it is while my pleasure centers are fully activated.

I'm pretty sure that speed can be reached if the parameters are set right. I make sure of that in AJE even. Players need to change the playstyle a bit: a small price to pay. But the inevitable aero overhaul is the same, why not do everything right?

[Kegereneku]

Interesting, I've never heard of a RL rocket engine that has a lower Isp at low atmospheric pressure than in high, the most I've seen is ones that are not as bad in atmo in exchange for being less good in vacuum but vac Isp is till greater than atmo Isp. Do you know of an example of such a design?

Er... I'm not sure if you got me wrong or if I got you right.

When I said "better" was more talking of thrust which vary much more than Isp which as you say have no reason to get worse.

So, normal you haven't heard of any.

In any case we usually don't use rocket engine at pressure they aren't worth using for (weight, thrust, along the difficulty of reigniting rocket), one of the lift-off engine we know that burn up to vacuum is the Space Shuttle Engine which in fact has an Isp greater than its OMS engines.

Here is one of the phenomena that influence nozzle shape.

http://en.wikipedia.org/wiki/Rocket_engine_nozzle#Aerostatic_back-pressure_and_optimum_expansion

It's important enough that we develop nozzle that change shape, and the Aerospike was supposed to be best at keeping the same thrust at any altitude.

http://en.wikipedia.org/wiki/Aerospike_engine

hope this clear thing out.